Conventionally, as the methods of developing the electrostatic latent image formed on the image carrier in an image forming apparatus using the electro-photographic method, the one-component developing system which uses only a toner as the developing agent and the two-component developing system which uses a toner and a carrier are known.
Generally, in the one-component developing system, the toner is charged by passing the toner through a regulating section that has a toner supporting member and a regulating plate that presses against that toner supporting member, a desired toner layer is formed and the electrostatic latent image is developed. Therefore, since the development is made in a state of close proximity between the toner supporting member and the image carrier, it is superior in dot reproducibility, and also, by forming a uniform toner layer, it is possible to obtain a uniform image without the generation of image irregularity that is caused by a magnetic brush in the two component development method. In addition, it is considered to be advantageous in terms of simplification of the apparatus, size reduction, and achieving low cost. However, on the other hand, because of the strong stress in the regulating section, the surface properties of the toner get altered thereby reducing the charge-receiving property of the toner, and the surfaces of the toner regulating member and the toner supporting member get contaminated due to the adhesion of the toner or of the external additive agents, and hence the property of applying charge to the toner gets reduced thereby causing the problem of fogging due to insufficient charging of the toner and the problem of contamination inside the apparatus. Therefore, there is the problem of the life of the development apparatus becoming short.
On the other hand, in the two-component development system, since the toner is charged by friction charging due to mixing the toner with a carrier, the stress is small, and this method is very advantageous regarding toner deterioration. In addition, even because surface area is large of the carrier which is the material applying electric charge to the toner, this method is relatively strong against contamination caused by toner or external additive agents, and this method is advantageous for making the life longer.
However, even when a two-component developer is used, the surface of the carrier does get contaminated by the toner and the external additive agents, the amount of charging of the toner gets reduced over a long time of use, and problems such as fogging or toner splashing occur, and the life can not be said to be sufficient, and a still longer life is desired.
In the Japanese Laid-Open Patent Application Publication No. S59-100471 is disclosed a development apparatus that suppresses the increase in the ratio of deteriorated carriers by replenishing in small quantities the carrier in the developer together with the toner or independently, and accordingly, the replacement of carrier is carried out by discharging the deteriorated developer whose charging property has gone down. Since the carriers are being replaced in this apparatus, it is possible to suppress to a constant level the reduction in the extent of charging of the toner due to carrier deterioration, and this method is advantageous in terms of obtaining a long life of the apparatus.
Further, in the Japanese Laid-Open Patent Application Publication No. 2003-215855 is disclosed a two-component developer having a toner in which are externally added particles having the property of being charged to a polarity opposite to the charging polarities of the carrier and the toner and a development method using this developer. In this method, it has been indicated that particles with opposite polarity charging property are added with the intention of acting as polishing material and spacer particles, and that there is the effect of suppressing deterioration due to the effect of removing the spent matters on the surface of the carrier. In addition, it is said that there is the effect of improving the cleaning in the image carrier cleaning section and of polishing the image carrier.
Further, in the Japanese Laid-Open Patent Application Publication No. H9-185247 is disclosed a so-called hybrid type development method in which only the toner in the two-component developer is made to be carried on to the toner-supporting member opposite the image carrier and the electrostatic latent image on the image carrier is developed. In the hybrid development method, image unevenness due to a magnetic brush is not generated, and hence the method has excellent dot reproducibility and image uniformity. In addition, this method has other features that are not present in normal two-component development methods such as there is no occurrence of transfer of the carriers to the image carrier (carrier consumption) because there is no direct contact between the image carrier and the magnetic brush, etc. In the hybrid development method, since the charging of the toner is done due to friction with the carrier, maintaining the charge applying property of the carrier is important in stabilizing the chargeability of the toner and maintaining good image quality over a long period.
However, in the development apparatus disclosed in the Japanese Laid-Open Patent Application Publication No. S59-100471, there are problems in the aspects of cost and environment because a mechanism for recovering the discharged carrier is necessary, and because the carrier becomes a consumable item. In addition, it is necessary to repeat the printing for a prescribed volume until the ratio of old to new carriers becomes stable, and it is not necessarily possible to maintain the initial characteristics. Further, in the Japanese Laid-Open Patent Application Publication No. 2003-215855, the amounts of consumption of the toner and the opposite polarity charging particles differ depending on the image area ratio, particularly when the image area ratio is small, the consumption of the opposite polarity charging particles adhered to the large non-image area becomes excessive, and there is the problem that the effect of suppressing the carrier deterioration in the development apparatus becomes lower. In addition, in the hybrid development method disclosed in the Japanese Laid-Open Patent Application Publication No. H9-185247, there is the problem that as the number of sheets printed increases, the surface of the carrier gets contaminated by toner and post-processing materials, and the charge applying property of the carrier decreases successively.